Then I relaised, that resising images on the fly might be the main reason of the slow down. This idea came to me on Friday afternoon. The end of the day was intense, but resizing images once and staving the sizes I need improved the speed. Still, the GPU is not 100% busy. Now I'm looking for Monday, to see the results!

At that point TemsorFlow was complaining that the CPU supports instructions that the binary was not built with. Before building TF from source, I decided to update cuda and other Nvidia software. After a dance following outdated tutorials, I got it done. Speed improved, but still, while CPUs were at 100%, GPUs were barely loaded.